In recent years, the need has been increasing for reducing production lead time. However, in an articulated (vertical or horizontal) robot used for welding or handling, the production needs to be stopped for several hours to replace a faulty mechanism element such as a motor and speed reducer for driving the robot, becoming a major problem.
When an element such as a motor and speed reducer becomes faulty, the frictional force rises to increase the motor drive force in most cases. Under the circumstances, if an unusual condition of the motor drive force can be recognized before the robot becomes completely inoperable, a mechanism element such as a motor can be replaced while the production line is nonoperating (e.g. on a day off, in the nighttime), thus reducing the effects on the production.
Thus, a method is known of detecting a failure of a robot focusing attention on fluctuation of a velocity waveform of a motor caused by a failure (refer to patent literature 1 for example). Another method is known of detecting a failure of a robot focusing attention on the difference between a power at the drive side (motor output power) and a power at the load side (a power obtained by a motion equation of a robot (refer to patent literature 2 for example).
The former method of determining a failure from fluctuation of a velocity waveform has the following problems. The first one is that velocity waveform data in a normal state as reference data needs to be measured and stored for each robot. The second one is that a robot needs to be operated with a reference operation pattern to determine a failure. Accordingly, failure determination naturally cannot be performed while the production line is operating because velocity waveform data of a motor while the production line is operating is different from that for failure determination. Meanwhile, even if the production line can afford time to perform failure determination (e.g. nonoperating production line), a robot needs to be operated with a reference operation pattern only for failure determination, which results in a cost for worker-hours for failure measurement. In a production field where the production line operates for a long time and severely in cutting cost, it is practically difficult to perform the above two points only for determining a failure of a robot.
In the latter method of determining a failure from the difference in power, the former problem is solved; reference data does not need to be measured or stored; and determination is possible while the production line is operating. However, a power at the load side cannot be necessarily calculated accurately although a power at the drive side can be. This is because calculation of a power by a motion equation of a robot produces an error unless parameters (e.g. mass, barycenter position, inertia) of a load (e.g. welding torch, handling tool, workpiece)are accurate. In patent literature 2, a load torque value is calculated from a motion equation, angle, angular velocity, and angular acceleration for a mass point model of the mechanism element including an attached load, and then the load torque value is multiplied by the angular velocity to determine a power at the load side.
For an industrial robot of recent years, accurate load parameters are requested for optimizing acceleration and deceleration and for a higher degree of precision in collision detection. Accordingly, a lot of robots are found equipped with a device for inputting load parameters or a function of automatically measuring load parameters. However, whether load parameters are input or an automatic measurement function is used depends on a user, and accurate load parameters are not necessarily input. A large difference between a set parameter and its actual parameter for a load results in a large error in a power at the load side, which can cause erroneous decision as a failure.
A power at the drive side increases not only because a friction increase due to a failure of a motor or speed reducer driving the robot increases the motor drive force. That is also because the motor drive force increases or decreases responding to an external force in a case such as where the robot contacts another object and a tensile force is exerted on a cable attached to the robot. However, in the former case where friction increases, the motor drive force increases as well to compensate the increased friction energy. Meanwhile, in the latter case where an external force is exerted, a power at the drive side does not necessarily increase. For example, a tensile force increase of the cable bears the gravitational force of the robot to possibly decrease a power at the drive side. Even in such a case, the difference between a power at the drive side and that at the load side increases, which can cause erroneous decision as a failure even if caused by other than a mechanism failure.
FIG. 11 is an explanatory drawing of the above-described case where a tensile force increase of a cable bears the gravitational force of a robot, showing an outline structure of a conventional welding robot system.
In FIG. 11, welding wire 101 as a consumable electrode is sent from wire spool 102 to welding torch 104 through torch cable 111 (shown by dotted lines) with a hollow structure by wire feeding motor 103. Welding power supply unit 105 applies given welding current I and welding voltage V between welding wire 101 and base material 107 (welded object) through welding torch 104 and welding tip 106 to generate arc 108. Further, welding power supply unit 105 controls wire feeding motor 103 to perform welding. Robot 109 holds welding torch 104 to position it to a welding start point (not shown) and moves welding torch 104 along a weld line (not shown). Such controlling of the entire robot is performed by robot control unit 110.
At this moment, a user often places, for example, jig 112 for hanging torch cable 111 from above for ensuring the feeding performance by maintaining the shape of welding wire 101, and for avoiding interference with surrounding objects. Here, torch cable 111 moves according to operation of robot 109. Consequently, jig 112 is usually made of an elastic body such as a spring and rubber. Accordingly, a force pulling upward is exerted on welding torch 104 and robot 109 holding it as well through torch cable 111 to bear the gravitational force of robot 109. Particularly, the tip of a robot to which welding torch 104 is attached is loaded with a motor with a small capacity, and thus such a change in tensile force of torch cable 111 is nonnegligible. Such jig 112 is selected, mounted, and exchanged by a user. Hence, using, mounting, or exchanging jig 112 can cause erroneous decision as a failure.